When operating a compressor, it is necessary to avoid the vibration phenomenon known as surging that is accompanied by pressure fluctuations or backflow within a compressor, from the point of view of protecting the compressor and the nearby equipment.
Surging can be prevented by controlling the actual pressure ratio of the compressor to be smaller than the limiting pressure ratio. The limiting pressure ratio is determined at each corrected RPM of the compressor. The corrected RPM is a value determined by the compressor actual RPM N, as well as by the specific heat ratio κ of the inlet gas, the gas constant R of the inlet gas, and the absolute temperature T of the inlet gas. Therefore, the corrected RPM varies in accordance with the temperature, composition, and the like of the inlet gas. In other words, when the temperature, composition, or the like of the inlet gas varies, the corrected RPM varies accordingly, and as a result, the limiting pressure ratio also varies.
Therefore, in Patent Document 1 below, a method is proposed for determining the limiting pressure ratio in accordance with the corrected RPM, by sensing the temperature of the inlet gas, and finding the corrected RPM in accordance with this temperature. Also, in Patent Document 1, a method is proposed for determining the limiting pressure ratio in accordance with the corrected RPM by sensing the temperature and density of the inlet gas, and finding the corrected RPM in accordance with these temperature and density. In addition, in Patent Document 1, a method is proposed for determining the limiting pressure ratio in accordance with the corrected RPM by sensing the temperature of the inlet gas, as well as sensing the composition of the inlet gas by gas chromatography, and finding the corrected RPM in accordance with these temperature and composition.